A photovoltaic cell is an electrical device formed of semiconducting material able to absorb electromagnetic energy generally from light and convert it directly into electrical energy. An individual photovoltaic cell has a distinct spectrum of light to which it is responsive; the particular spectrum is primarily a function of the material forming the cell. Photovoltaic cells that are responsive to the visible light spectrum are commonly referred to as solar cells.
Individually, any given photovoltaic cell is capable of generating only a relatively small amount of power. For example, some solar cells generate power in the range of 0.004 to 0.02 watts/cm.sup.2 surface area. One means of increasing the power generated by a photovoltaic cell is to fabricate a monolithic, tandem solar cell. This type of photovoltaic cell includes at least an upper cell and a lower cell that are stacked together. The upper cell, identified as the upper tandem subcell circuit, is formed of semiconducting material that is responsive to light over a first range of wavelengths but is transparent to light at a second range of wavelengths. The lower cell, identified as the lower tandem subcell circuit, is formed of material responsive to light at a second range of wavelengths. An advantage of these monolithic cells is that they are able to absorb a relatively wide range of incident light and efficiently convert it into electrical energy. This type of photovoltaic cell is capable of generating anywhere from 0.0108 to 0.0403 watts/cm.sup.2, twice the power of other photovoltaic cells. U.S. Pat. Nos. Re. 31,968; 4,392,451; 4,523,051; 4,684,761; 4,680,422; 4,795,501; 4,867,801; 5,021,099; and 5,078,804, incorporated herein by reference, disclose how such tandem cells can be manufactured. U.S. Pat. No. 4,867,801 discloses how a triple tandem cell with three photovoltaic regions can be fabricated. U.S. Pat. Nos. 5,091,018 and 5,096,505 further disclose how to manufacture tandem cells wherein the upper and lower cells are attached to a separate substrate.
To date, monolithic tandem cells have been fabricated so that the upper and lower subcell circuits are series connected to each other, because it has proven difficult to provide an isolation layer that can prevent electron flow between the upper and lower tandem subcell circuits. Furthermore, it has been an equally hard challenge to provide transparent backplane connection to the surfaces of the individual subcell circuits that interface each other. Such connection has been especially difficult with regard to the lower tandem subcell circuit. Metals, for example, cannot be used to provide such connection because, if they are formed on the monolithic cell in a layer thin enough to be transparent, they typically do not have sufficient conductivity to allow the current developed by the cell to readily flow therethrough. Finding a transparent semiconducting material that can serve as a backplane connector has proven to be an equally difficult task. Thus, current tandem cells are formed in a series-connected arrangement wherein the current that develops in each subcell circuit flows through the companion subcell circuits
A disadvantage of this connection is that the individual subcell circuits forming the monolithic cell must be current matched. In other words, the individual subcell circuit cells must be fabricated so that the same current flows through each cell. If the individual subcell circuits are not so arranged, then the subcell circuit that produces less current will function as a power sink instead of power source, reducing the overall power output of the monolithic cell. Alternatively, the current out of a monolithic cell is limited by the current generated by the subcell circuit that generates the smallest current. This limitation similarly reduces the overall power that is generated by the cell.
Machining monolithic photovoltaic cells has proven to be a difficult task because fabricating the individual subcell circuits so that they are current matched is difficult. Once formed, maintaining the match is difficult because, when the monolithic cell is exposed to ionizing radiation, the semiconducting material forming the individual subcell circuits starts to break down, reducing the current generated. The upper and lower tandem subcell circuits typically break down at different rates. This causes the power generated by the individual subcell circuits to likewise change at different rates. Once this breakdown occurs, the individual cells are no longer current matched and the power produced by the monolithic cell, as a whole, begins to fall appreciably.